Strictly from a cost-savings standpoint, it really isn't any cheaper than purchasing 2-liter bottles at the store.
It is definitely fun to use so your son would probably enjoy it if all you care about is him having fun with it.
I don't personally own one but have a brother-in-law who does (they have kids) and they use it every once and a while.
I am picky about soda...I can drink Coke or Pepsi, but I don't like any store brands...and I don't like Sodastream's cola. Their Dr. Pepper equivalent isn't bad. I'm not a root beer fan of any kind so I can't say if that is good or not!
I like the fact that you don't have to use the syrups. You can carbonate say, 100% no sugar added juice for example.
I totally agree that if your kid is a lover of root beer, the root beer making kit would be awesome for him!